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Ruth Durrer

Publications and source records attributed to Ruth Durrer.

At least 181 records · Page 10Linked to original sources
arXiv2005

Limits on stochastic magnetic fields: A defense of our paper [1]

In their recent paper ``Faraday rotation of the cosmic microwave background polarization by a stochastic magnetic field'', Kosowsky et al. Phys.Rev. D71, 043006 (2005) have commented about our paper [C.Caprini and R.Durrer, Phys. Rev. D65, 023517 (2002)], in which we derived very strong limits on the amplitude of a primordial magnetic field from gravitational wave production. They argue that our limits are erroneous. In this short comment we defend our result.

astro-ph↗
arXiv2005

Why do we live in 3+1 dimensions?

In the context of string theory we argue that higher dimensional Dp-branes unwind and evaporate so that we are left with D3-branes embedded in a (9+1)-dimensional bulk. One of these D3-branes plays the role of our Universe. Within this picture, the evaporation of the higher dimensional Dp-branes provides the entropy of our Universe.

hep-th↗
arXiv2004

The Cosmic Microwave Background and Helical Magnetic Fields: the tensor mode

We study the effect of a possible helicity component of a primordial magnetic field on the tensor part of the cosmic microwave background temperature anisotropies and polarization. We give analytical approximations for the tensor contributions induced by helicity, discussing their amplitude and spectral index in dependence of the power spectrum of the primordial magnetic field. We find that an helical magnetic field creates a parity odd component of gravity waves inducing parity odd polarization signals. However, only if the magnetic field is close to scale invariant and if its helical part is close to maximal, the effect is sufficiently large to be observable. We also discuss the implications of causality on the magnetic field spectrum.

astro-ph↗
arXiv2004

Cosmological perturbation theory

This is a review on cosmological perturbation theory. After an introduction, it presents the problem of gauge transformation. Gauge invariant variables are introduced and the Einstein and conservation equations are written in terms of these variables. Some examples, especially perfect fluids and scalar fields are presented in detail. The generation of perturbations during inflation is studied. Lightlike geodesics and their relevance for CMB anisotropies are briefly discussed. Perturbation theory in braneworlds is also introduced.

astro-ph↗
arXiv2004

Testing braneworlds with the binary pulsar

In this letter we calculate the emission of gravity waves by the binary pulsar in the framework of five dimensional braneworlds. We consider only spacetimes with one compact extra-dimension. We show that the presence of additional degrees of freedom, especially the 'gravi-scalar' leads to a modification of Einstein's quadrupole formula. We compute the induced change for the binary pulsar PSR 1913+16 where it amounts to about 20% which is by far excluded by present experimental data.

hep-th↗
arXiv2003

Primordial Magnetic Fields and Causality

We discuss the implications of causality on a primordial magnetic field. We show that the residual field on large scales is much more suppressed than usually assumed and that a helical component is even more reduced. Due to this strong suppression, even maximal primordial fields generated at the electroweak phase transition can just marginally seed the fields in clusters, but they cannot leave any detectable imprint on the cosmic microwave background.

astro-ph↗
arXiv2003

CMB anisotropies from vector perturbations in the bulk

The vector perturbations induced on the brane by gravitational waves propagating in the bulk are studied in a cosmological framework. Cosmic expansion arises from the brane motion in a non-compact five-dimensional anti-de Sitter spacetime. By solving the vector perturbation equations in the bulk, for generic initial conditions, we find that they give rise to growing modes on the brane in the Friedmann-Lema\^ıtre era. Among these modes, we exhibit a class of normalizable perturbations, which are exponentially growing with respect to conformal time on the brane. The presence of these modes is, at least, strongly constrained by the current observations of the cosmic microwave background (CMB). We estimate the anisotropies they induce in the CMB, and derive quantitative constraints on the allowed amplitude of their primordial spectrum. Our results provide stringent constraints for all braneworld models with bulk inflation.

hep-th↗
arXiv2003

Bias and the power spectrum beyond the turn-over

Threshold biasing of a Gaussian random field gives a linear amplification of the reduced two point correlation function at large distances. We show that for standard cosmological models this does not translate into a linear amplification of the power spectrum (PS) at small k. For standard CDM type models this means that the``turn-over'' at small k of the original PS disappears in the PS of the biased field for the physically relevant range of the threshold parameter. In real space this difference is manifest in the asymptotic behaviour of the normalised mass variance in spheres of radius R, which changes from the ``super-homogeneous'' behaviour to a Poisson-like behaviour.This qualitative change results from the intrinsic stochasticity of the threshold sampling. While our quantitative results are specific to the simplest threshold biasing model, we argue that our qualitative conclusions should be valid generically for any biasing mechanism involving a scale-dependent amplification of the correlation function. One implication is that the real-space correlation function will be a better instrument to probe for the underlying Harrison Zeldovich spectrum in the distribution of visible matter, as the characteristic asymptotic negative power-law ξ(r) \sim -r^{-4} tail is undistorted by biasing.

astro-ph↗
arXiv2002

Gauge invariant cosmological perturbation theory for braneworlds

We derive the gauge invariant perturbation equations for a 5-dimensional bulk spacetime in the presence of a brane. The equations are derived in full generality, without specifying a particular energy content of the bulk or the brane. We do not assume Z_2 symmetry, and show that the degree of freedom associated with brane motion plays a crucial role. Our formalism may also be used in the Z_2 symmetric case where it simplifies considerably.

hep-th↗
arXiv2002

Frontiers of the Universe: What do we know, what do we understand?

In this talk I summarize the topics addressed during the conference. We have discussed the most relevant cosmological observations of the last say 10 years and their implications for our understanding of the Universe. My finding throughout this meeting has been that it is amazing how our knowledge of cosmological parameters has improved during the last years. It comes as a surprise to many, that the simplest inflationary model of adiabatic perturbations agrees well with present data. But it is frustrating that we have not made any progress in identifying the dark matter, in contrary, we have supplemented it by what we call the 'dark energy', a gravitationally and esthetically repulsive component which is even more mysterious than the dark matter. We have no understanding about the origin of these two most abundant energy components of the present Universe.

astro-ph↗
arXiv2002

Clarifying perturbations in the ekpyrotic universe

In this note I try to clarify the problem of perturbations in the ekpyrotic universe. I write down the most general matching conditions and specify the choices taken by the two debating sides. I also bring up the problem of surface stresses which always have to be present when a transition from a collapsing to an expanding phase is made.

hep-th↗
arXiv2001

Gravitational wave production: A strong constraint on primordial magnetic fields

We compute the gravity waves induced by anisotropic stresses of stochastic primordial magnetic fields. The nucleosynthesis bound on gravity waves is then used to derive a limit on the magnetic field amplitude as function of the spectral index. The obtained limits are extraordinarily strong: If the primordial magnetic field is produced by a causal process, leading to a spectral index $n\ge 2$ on super horizon scales, galactic magnetic fields produced at the electroweak phase transition or earlier have to be weaker than $B_\la \le 10^{-27}$Gauss! If they are induced during an inflationary phase (reheating temperature $T\sim 10^{15}$GeV) with a spectral index $n\sim 0$, the magnetic field has to be weaker than $B_\la \le 10^{-39}$Gauss! Only very red magnetic field spectra, $n\sim -3$ are not strongly constrained. We also find that a considerable amount of the magnetic field energy is converted into gravity waves. The gravity wave limit derived in this work rules out most of the proposed processes for primordial seeds for the large scale coherent magnetic fields observed in galaxies and clusters.

astro-ph↗
arXiv2001

The theory of CMB anisotropies

This is a review of the theory of CMB anisotropies, an updated version of a course given at the troisieme cycle de la Suisse Romande. An introduction to gauge invariant cosmological perturbation theory is given and the theory CMB anisotropies is develiped in this context. Simple analytical approximations for the acoustic peak positions for adiabatic and isocurvature perturbations are derived. Silk damping is discussed by an analytic approximation. A short description of the present status of observations and parameter estimation followed by a critical discussion terminate the review. The full system of differential equations for CMB anisotropies and polarization needed in a numerical treatment is also developed and given in an appendix.

astro-ph↗
arXiv2001

Dynamical Instabilities of the Randall-Sundrum Model

We derive dynamical equations to describe a single 3-brane containing fluid matter and a scalar field coupling to the dilaton and the gravitational field in a five dimensional bulk. First, we show that a scalar field or an arbitrary fluid on the brane cannot evolve to cancel the cosmological constant in the bulk. Then we show that the Randall-Sundrum model is unstable under small deviations from the fine-tuning between the brane tension and the bulk cosmological constant and even under homogeneous gravitational perturbations. Implications for brane world cosmologies are discussed.

hep-th↗
arXiv2000

CMB anisotropies in pre-big bang cosmology

We present an alternative scenario for cosmic structure formation where initial fluctuations are due to Kalb-Ramond axions produced during a pre-big bang phase of inflation. We investigate whether this scenario, where the fluctuations are induced by seeds and therefore are of isocurvature nature, can be brought in agreement with present observations by a suitable choice of cosmological parameters. We also discuss several observational signatures which can distinguish axion seeds from standard inflationary models.

astro-ph↗
arXiv2000

CMB Signatures of a Primordial Magnetic Field

A primordial stochastic magnetic field will induce temperature and polarization fluctuations in the cosmic microwave background. We outline a calculation of the resulting fluctuation power spectra and present numerical results.

astro-ph↗
arXiv2000

Skewness as a probe of non-Gaussian initial conditions

We compute the skewness of the matter distribution arising from non-linear evolution and from non-Gaussian initial perturbations. We apply our result to a very generic class of models with non-Gaussian initial conditions and we estimate analytically the ratio between the skewness due to non-linear clustering and the part due to the intrinsic non-Gaussianity of the models. We finally extend our estimates to higher moments.

astro-ph↗